Drop the necessity to add ${PORTSDIR} to dependency line
Modify make describe to automatically prepend ${PORTSDIR} if the path for the
port is not absolute
Checked with poudriere, portmaster, portupgrade
PR: 203685
Exp-run by: antoine
Differential Revision: https://reviews.freebsd.org/D3866

Change the way USES is handled:
- Loop over USES twice, once to define all *_ARGS variables and once to
include Uses/*.mk. This allows all Uses/*.mk to examine arguments given
to other USES entries.
- Always define *_ARGS (possibly empty) and replace commas with spaces.
Similar for _USES_POST.
Adjust all Uses/*.mk:
- defined(u_ARGS) becomes !empty(u_ARGS)
- Eliminate helper variables like _*_ARGS=${*_ARGS:C/,/ /g}
- Some Uses/*.mk used ":" as argument separator instead of ",", but no port
used this form
- Uses/cran.mk: remove unused variable VALID_ARGS and USES+=fortran which
has no effect
- Uses/twisted.mk: simplify handling of the case where neither "build" nor
"run" arguments have been specified
PR: 193931
Exp-run by: antoine
Approved by: portmgr (antoine)

Convert all :U to :tu and :L to :tl
Since FreeBSD 8.4 and FreeBSD 9.1 make(1) do support :tu and :tl as a
replacement for :U and :L (which has been marked as deprecated)
bmake which is the default on FreeBSD 10+ only support by default
:tu/:tl a hack has been added at the time to support :U and :L to ease
migration. This hack is now not necessary anymore
Note that this makes the ports tree incompatible with make(1) from
FreeBSD 8.3 or earlier
With hat: portmgr

- Convert USE_QMAIL_RUN, USE_QMAIL_BUILD and WANT_QMAIL into USES feature
bapt contributed to the qmail.mk as well
- Trim headers
USES= qmail:run will add qmail into RUN_DEPENDS
USES= qmail:build will add qmail into BUILD_DEPENDS
USES= qmail[:both] will add qmail into both RUN and BUILD DEPENDS
USES= qmail:vars will set QMAIL_PREFIX
With hat: portmgr